174 Evolution and Adaptation 
is probably attempting to show is that the material for the 
further action of sexual selection is already given; but the 
question may well be asked, if the external conditions have 
done so much, why may they not have gone farther and pro- 
duced the entire result ? 
Darwin makes the following suggestion to account for those 
cases in which the female is the more highly colored : — 
“A few exceptional cases occur in various classes of 
animals, in which the females instead of the males have 
acquired well-pronounced secondary sexual characters, such 
as brighter colors, greater size, strength, or pugnacity. 
With birds there has sometimes been a complete transposi- 
tion of the ordinary characters proper to each sex; the 
females having become the more eager in courtship, the 
males remaining comparatively passive, but apparently select- 
ing. the more attractive females, as we may infer from the 
results. Certain hen birds have thus been rendered more 
highly colored or otherwise ornamented, as well as more 
powerful and pugnacious than the cocks; these characters 
being transmitted to the female offspring alone.” 
Then follows immediately the discussion as to whether a 
double process of sexual selection may not be supposed to go 
on at the same time. “It may be suggested that in some 
cases a double process of selection has been carried on; that 
the males have selected the more attractive females, and the 
latter the more attractive males. This process, however, 
though it might lead to the modification of both sexes, would 
not make the one sex different from the other, unless indeed 
their tastes for the beautiful differed ; but this is a supposition 
too improbable to be worth considering in the case of any 
animal, excepting man. There are, however, many animals 
in which the sexes resemble each other, both being furnished 
with the same ornaments, which analogy would lead us to 
attribute to the agency of sexual selection. In such cases 
it may be suggested with more plausibility, that there has 
